The National Maritime Center posted a bulletin to its website Nov. 13, 2018, to inform mariners applying for a Merchant Mariner Credential that they must list their occupation as Merchant Mariner on their TWIC applications. The Coast Guard relies on data provided by the Transportation Security Administration when evaluating MMC applications.
Source: USCG
Link: 11/14/2018: Merchant mariner credentials and TWIC enrollment